dfa: make find_pred return NULL for an invalid predicate

This could never happen when invoked via grep, but could have triggered
a bug if dfa.c's find_pred function were invoked by some other program.
* src/dfa.c (find_pred): Return NULL for an invalid predicate.
* tests/invalid-char-class: New file to test for this.
* tests/Makefile.am (TESTS): Add that new file name to the list.
This addresses http://debbugs.gnu.org/18631

1 files changed, 2 insertions, 3 deletions

diff --git a/src/dfa.c b/src/dfa.c
index 6c46919b..8901f69d 100644
--- a/src/dfa.c
+++ b/src/dfa.c
@@ -971,9 +971,8 @@ find_pred (const char *str)
   unsigned int i;
   for (i = 0; prednames[i].name; ++i)
     if (STREQ (str, prednames[i].name))
-      break;
-
-  return &prednames[i];
+      return &prednames[i];
+  return NULL;
 }
 
 /* Multibyte character handling sub-routine for lex.